p–n Transition in Thermoelectric Semiconductor Eskebornite

Abstract

Eskebornite (CuFeSe2) is a I–III–VI2 semiconductor with a tetragonal crystal structure, known for its intriguing electrical and magnetic properties. However, experimental studies on this material remain scarce. In this study, Ni-doped eskebornite, Cu1−xNixFeSe2 (x = 0.02–0.06), was synthesized via solid-state methods by substituting Ni2+ for Cu+. Mechanical alloying was employed to prepare the compounds, followed by hot pressing. X-ray diffraction analysis revealed the eskebornite phase alongside a minor secondary phase, identified as penroseite (NiSe2) with a cubic crystal structure. Thermoelectric properties were measured over the temperature range of 323–623 K. The Seebeck coefficient exhibited p-type behavior at low temperatures but transitioned to n-type at higher temperatures, indicating a temperature-dependent p–n transition due to changes in the dominant charge carriers. With increasing Ni doping, the Seebeck coefficient increased positively at low temperatures and negatively at high temperatures, with the p–n transition temperature shifting to lower values. Electrical conductivity decreased with higher Ni doping levels, while its positive temperature dependence became more pronounced, reflecting non-degenerate semiconductor behavior. Thermal conductivity showed a negative temperature dependence but increased with higher Ni content. The highest thermoelectric performance was observed for Cu0.98Ni0.02FeSe2, achieving ZTp = 0.30 × 10–3 at 523 K, and for Cu0.94Ni0.06FeSe2, achieving ZTn = 0.55 × 10–3 at 623 K, where ZTp and ZTn represent the dimensionless figure of merit for p-type and n-type thermoelectric materials, respectively.